SEM sample preparation

Transcription

1 SEM sample preparation 6 th CEMM workshop Maja Koblar, Sc. Eng. Physics

2 Outline Before SEM characterization Preparation of: bulk material hard and soft powders Mounting: holders and adhesives Coating: sputtering and evaporation Thin film growth

3 Before SEM characterization

4 Before you start the preparation? Is the sample vacuum compatible? Interested in morfology (shape of the particles or the microstructure, grain size, grain shape,..) Do you want to know the chemistry? Discuss different options of preparation

5 Sample as small as possible Less is more! Image: providr.com Image: more.com The same goes for SEM samples Why? Less pumping, less outgassing. Powder need special handling, otherwise they may get loose and fly off the holder. If magnetic it can damage the EM. We usually look at a very small area.

6 Preparation of bulk: hard material Composites, metals, rocks, ceramics, glass,

7 don't shake Dry and clean Use solvents depending on the sample. If too big cut it with the leaf saw. Surface observation of the material as it is Surface topography (SE) Imaging SE and BSE Some samples are smooth and have a mirror surface. Mounting securely on a SEM stub Make pathway for e- on the adhesive layer and on the sample: SEM If conductive, use high voltage, if nonconductive use low voltages or a variable pressure SEM. coating Make a path to ground. etching SEM SEM coating SEM Avoid microanalysis.

8 Cutting Make sure that the selected part is representing the whole sample. Mounting, embedding in resin Surface observation and open surface of the sample s inner structure (for BSE, EDS, WDS, EBSD) grinding polishing cleaning, mounting on a SEM stub SEM coating etching Storing the samples in a vacuum dryer. SEM SEM coating SEM

9 Unpolished: Topography (SE) Fractures or cross sections, exposure by breaking, cutting, cleaving, snapping and pulling. Polished to a smoth surface: Topography (SE) Compositional imaging (BSE) Microanalysis

10 Preparation of bulk: soft and firm material Biological samples, paper, polymers, foams, gels, wood, food,

11 fixation Fixation stops cellular processes and aims to preserve the specimens as close as possible to its natural state. chemical vapour Surface observation of the material as it is critical point drying chemically dry freeze dry The sample is fixated, dyhidrated and dry Mounting on a SEM stub SEM coating etching Storing the samples in a vacuum dryer. SEM SEM coating Sometimes charging can be a problem

12 Surface observation of samples inner sturture, cross sections and thin sections Using: Scalpels, FIB, Ultramicrotome, Image: SCAN Image: embalmers.com Image: Goldstein, fiber Images: Nanocenter, FIB

13 Preparation of powders Powders, colloids and magnetic particles

14 Large particles: Diameter above 10 mm. Image: Jeol Particles diameter > 10 mm: Use carbon holders or aluminium holders Place a drop of carbon paint and spread the drop to form a layer Wait for the paint to evaporate to near dryness Collect a small amount of sample using a make sure that it is representable. While the paint is still tacky, place the particles: With forceps and binocular microscope Droop them the momentum from falling will embed the particles into the paint Gently tab the stub on the table to remove excess sample. Blow away excess sample using air duster. If non conductive, use low voltage or variable pressure SEM or coat the sample.

15 Small particles: Diameter between 5 μm and 10 mm. 10 mm > particle diameter > 5 μm: Use carbon holders or aluminium holders Place a double sided carbon / copper tape Collect a small amount of sample using a transfer pipet, make sure that it is representable. Apply a thin layer of powder on the tape. Gently tab the stub on the table to remove excess sample. Blow away excess sample using air duster. If non conductive, use low voltage or variable pressure SEM or coat the sample. Image: Jeol

16 Smaller particles: Diameter below 5 μm Particle diameter < 5 μm: Use carbon holders or aluminium holders Add small amount of powder in solvent, make sure that is representable (particle settling). Put it in the ultrasound to disperse the particles well. Place a small drop of suspension on a flat surface, again make sure that is representable (particle settling). Wait till it dries. Coat the sample with carbon to provide an additional adhesion. Dispersion can also bi made with Freon. The faster the solution evaporates, the less particle aggregation will occur.

17 BSE or inner surface BSE imaging: The powder should be hot-mounted in resin or cold-mounted (embedded) in epoxy resin, polymerised Polished to a smooth mirror surface If non conductive, use low voltage or variable pressure SEM or coat the sample. Do not use silver paint, due to the size of the silver particles and the high Z

18 Magnetic materials require special care! Magnetic materials can be investigated, but care is needed so that the powder is not attracted to the objective lens pole piece where it could disrupt the electron optics. Image: Špela K7

19 Mounting: holders and adhesives

20 Holders Mounting stubs are made of: Aluminium: has good conductivity, low cost, easy shaping for SE and BSE (topography and compositional) for semi-quantitative (standardless) EDS Carbon: for particles and powder for EDS, BSE made with epoxy : The upper surface polished for BSE, ESD, WDS

21 Adhesive Must firmly fix the sample (no mechanical drift when tilted or thermal drift when irradiated by the beam). Must be vacuum compatible and no outgassing. No super glues outgassing Double sided tape Carbon or copper Fast drying paint Carbon or silver Plastic conductive carbon cement For irregular shapes Two component silver filed epoxy Needs to be heated!

22 Conductive coating Charging leads to variations in surface potential: deflected secondary e-, increase secondary emission e-, Ddflection of electron beam, spurious x-ray signal

23 What material should I coat with? Au: standard for sputtering, easy to coat, inert, stable under electron beam Au/Pd: all the advantages of Au bus smaller grain size Pt: highly inert, very conductive but medium grain size Cr: smallest grain size, but oxides quickly C: best for X-ray microanalysis and EBSD

24 Polimer coated with Balzers Au C HIGH MAG Au C LOW MAG

25 Cheramics coated with PECS Pt Cr C Pt Cr C

26 Coating the sample Balzers

27 Coating the sample - PECS

28 EVAPORATION The source is heated directly or indirectly until the point is reached where it efficiently sublimes or evaporates. The atoms or molecules start to leave the surface of the source and travel in more or less straight path until they reach another surface (substrate, wall). Since these surfaces are at lower T, the molecules will transfer there energy to the substrate, lower their T and condense. Other than pressure and temperature, the placement of the heater, source and substrate are important factors

29 Vapor pressure of elements Below the vapour pressure surface evaporation is faster than condensation, above it is slower. The vapor pressure of any substance increases non-linearly with temperature according to the Clausius- Clapyeron equation: Mass evaporation rate: dp = ΔH(T) dt TΔV Γ e = 5, M T P g v cm 2 s P pressure, T temperature, H entropy, V specific volume M molar mass P v vapor pressure

30 Vapor pressure [Torr] Evaporation rate for aluminium Al: M=27 g/mol P v = 10-7 bar = 10-4 Torr 980 C: Temperature [ C] Vapor pressure [bar] Γ e = 5, g cm 2 s = 9, g cm 2 s P v = 10-5 bar = 10-2 Torr 1220 C: Γ e = 5, g cm 2 s = 8, g cm 2 s

31 Sources of impurity What can effect film purity? Contamination of source materials- use high purity of source material Contamination of the heater - use material with low diffusion Residual gas in the chamberbetter vacuum, higher deposition rate (Image: wiki) (Image: lasp)

32 SPUTTERING Instead of using heat to eject material from a source, we can bombard them with high speed particles. The sputtering gas bombards the target and sputters off the material we would like to deposit. Once ejected, these atoms (or molecules) can travel to a substrate and deposit as a film.

33 Sputtering yield S [atoms/ions] Number of ejected (sputtered) atoms, molecules from target divided with the incident particle, ions S = Number of sputtered atoms Number of incident ions S depends on: 1. type of target atom and the binding energy 2. relative masses (of ions and atoms) 3. angle of incidence of ions and kinetic energy

34 Targets on SCD and PECS Magnetron target (Au, Pt) demonstrating racetrack erosion profile PECS targets (Cr, Au/Pd, C and Pt)

35 Physical mechanisms of thin film growth (Video: Barna, 1967)

36 Thickness measurment Gravimetric method Measure substrate weight before and after coating Calculate thickness from known substrate dimensions Not real-time but surprisingly accurate Stylus method profilometer or AFM A stylus is drawn across a step in the film Scratching can occur Needs calibration Can do repeated measurements Not real-time (Image: wiki)

37 Thickness measurment Specific oscillation frequency Expose one side of wafer to the vapor As the vapor coats the wafer the oscillation frequency changes Different parameters, proper calibration, quartz quality and proper usage Power supply Target (C,Au/Pt, Pt or Cr) shutter SAMPLE Sensor shutter oscillator BNC cable feedthrough vacuum crystal sensor

38 Take home information Before SEM characterization Check recent published papers Preparation Make sure that the samples are firmly attached Mounting: holders and adhesives Different holders (carbon, aluminum) Different adhesives, always wait for the adhesives to dry, check if it has to be heated Coating: sputtering and evaporation Always make ground (electrostatic charging) Choose the coating depending on the imaging and sample, for BSE use carbon, for biological specimens, polimers use higher Z. Thin film growth Different ways how the film growths Be sure that the surface image is not the coating, use some uncoated sample to check

39 Slovene Society for Microscopy

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